There has been an increased recent demand for styrene resins of reduced flammability. Various methods have been proposed to make styrene resins, including high-impact polystyrene, styrene-acrylonitrile-butadiene copolymers, styrene-methyl methacrylate-butadiene copolymers, and the like, less flammable. For example styrene resins having a reduced tendency to ignite and propagate flame in the absence of an external heat source have been prepared by adding an organic halide and an inorganic flame retardant, such as antimony trioxide. However, the use of such flame-retardants, especially materials such as antimony trioxide which must be introduced in particulate or crystaline form because they do not melt or fuse at temperatures at which styrene resins are usually worked, is likely to lower the toughness of the polymers markedly and to diminish or adversely affect desirable physical properties inherent in the resins. Even with the use of machines having a high mixing efficiency to work styrene resins, no appreciable improvement has been achieved. Also, flame-retarded styrene resins exhibit much diminished release properties during molding. This imposes a substantial limitation on the degree of freedom in designing, molding and productivity which are advantageous features of the styrene resins which do not contain flame retardants.
Organic halides are very effective in providing the polymers with a reduced tendency to ignite and burn in the absence of an external heat source, but have a tendency to make the polymers drip while they are burning, and, in order to achieve flame retarded compositions in the range of V-1 to V-0 provided under Subject 94 by Underwriter's Laboratories, Inc. (hereinafter referred to as UL-94), a large amount of halide must be added. The use of antimony compounds in combination with the halides is known to be effective in obviating such a defect. However, even with the addition of the antimony compounds, the polymers tend to drip as the thickness of polymer moldings becomes smaller, and it is very difficult to bring the polymers to conform to the range of V-1 to V-0 under UL 94. A solution to this problem has long been desired and is available in accordance with this invention by substituting p-methylstyrene polymers for styrene polymers.
The tests employed herein to demonstrate suppressed ignition properties and burning accurately demonstrate the ignition or burning characteristics of the polymers when exposed to small scale ignition sources according to the standards specified. It is well recognized by those skilled in the art and it should be clearly understood that all known organic polymers will burn when subjected to a sufficiently intense heat source whether or not they contain a fire-retardant additive. "To drip" or "dripping" referred to hereinafter, according to the vertical burning tests under UL-94, means to drip particles from a specimen during the application of a test flame or after the removal of the flame. Failure to achieve a V-1 or V-0 rating results when the dripping particles ignite a piece of cotton held under the specimen, regardless of whether or not the particles were flaming.
As can be seen from the above, a flame-retardant resin composition using substantially lower amounts of flame-retardant would be advantageous considering not only the physical properties of the end product but also cost effectiveness.